The images shown on the index page illustrate several of the experimental approaches used in my institute.
They represent data taken from real experiments.
Time-frequency plot of EEG responses
to visual stimuli
The EEG reflects potential changes that occur
in a coherent manner in populations of
cortical neurons. These signals are recorded
non-invasively by electrodes put on the scalp.
Modern techniques allow to analyse how the
frequency spectrum changes over time in
response to stimulus presentation (at time
zero). An increase of spectral power occurs in
the gamma-band (30-100 Hz) several
hundred milliseconds after a target stimulus
has appeared on a screen in front of the
subject.
Gamma oscillation in a multiunit response
Using microelectrodes (see below), action potentials
(spikes) can be recorded from small clusters of nerve
cells located in the cortex of an experimental animal.
When activated by an appropriate sensory stimulus,
the cells engage in coherent bursts of spikes (black
"needles" marked by red arrowheads). These bursts
occur at rather regular intervals, reflecting an
oscillatory process in the local network. Frequently, the
temporal interval between the bursts is on the order of
20ms, yielding an oscillation frequency around 50 Hz.
Placement of microelectrodes in the cortex guided by a map of orientation columns
The left part of the figure show a microscope view of an exposed
part of visual cortex in an anesthetized animal. The curved lines
correspond to blood vessels at the cortical surface. Using optical
imaging, the preferred orientation of the nerve cells can be
determined at each spot in the cortical map. Appropriate
superposition of the data yields a colour-coded map of the
orientation bands (right panel). This map can be used as a guide
for inserting microelectrodes (left) into columns with a particular
orientation. If multiple columns with similar orientation preference
are recorded simultaneously, neuronal oscillations are often found
to be correlated across spatially separate sites. We employ this
approach to study neural synchrony at the cellular level.
Functional magnetic resonance imaging (fMRI) of brain areas
Active regions of the human brain can be studied non-invasively using fMRI.
The method reveals areas that show increased oxygen consumption as a
consequence of enhanced neural activation. This allows to localize - with a
precision of few millimeters - neural assemblies involved in particular
perceptual, cognitive or motor tasks. We apply this method to study
activation of brain regions in the context of perceptual selection, attention
and conscious awareness. The figure on the right shows a prefrontal area
(orange spot) that is more strongly activated during conscious perception of
a visual stimulus as compared to a control condition where the same
physical stimulus appears on the retina but is not consciously perceived.
